Acreage to undergo surface coal mining requires a permit to ensure that mining will not create an environmental waste drainage problem. The site must be tested using an acid-base accounting (ABA) method for prediction of post mining water quality. Current methods may overestimate the potential acidity or alkalinity depending on the components present in the overburden. Needs exist for the development of an improved ABA method for analysis of coal mine overburden. U.S. Pat. Nos. 5,204,270 and 5,285,071 are incorporated herein by reference.
The existing methods compare total potential acidity and alkalinity. If the comparison shows an expected acid drainage, the mine is not permitted to be established.
The current acid accounting method involves prediction of total acidity based on analysis of the total sulfur content and assumes that all sulfur will be converted to sulfate with a corresponding production of acid.
The present base accounting method involves prediction of total alkalinity based on analysis of total carbonate and assumes that the neutralizing effect of all carbonates will be realized.
Recent work has been completed to compare "improved" variations of wet chemical back titration methods to obtain a neutralization potential for mine overburdens. These methods were evaluated in a recent journal article "Neutralization Potential of Overburden Samples Containing Siderite" by J. Skousen et al. in the Journal of Environmental Quality, Volume 26, no. 3, May-June, 1997 which compared the results of 31 samples all analyzed by three different laboratories. The methods include (1) the Sobek method, (2) a method where the sample is boiled for 5 minutes, (3) a method similar to (2) but includes a filter step and treatment with hydrogen peroxide before back titration, and (4) a modified Sobek method that includes the addition of hydrogen peroxide after the first hand titration. The results showed wide variation in neutralization potential among the four methods used and among the three laboratories producing the results. Clearly, a better method is required and will be welcomed by the agencies requiring these tests.
All sulfur forms in overburden do not contribute to acidity upon weathering. Predicting acidity based on the total sulfur content may result in a higher value than the real acidity, denial of many mining permits, and the loss of jobs.
All metal carbonates found in overburdens do not contribute to an overall neutralizing effect of acids. Use of total carbonate content may overestimate the overburden neutralizing capacity. This has resulted in mining permits being denied in cases where the acidity/alkalinity are too close to predict drainage quality. A better ABA method may open some of this acreage for mining.
The Department of Energy (DOE) is interested in a thermal method for analysis of overburden. Work toward a method is described in a paper entitled "Evolved Gas Analysis--A Method for Determining Pyrite, Marcasite, and Alkaline Earth Carbonates" by Hammack in Proceedings: 204.sup.th National Meeting of the ACS, Wash. D.C., Aug. 23-28, 1992 and a paper entitled "Evolved Gas Analysis--A New Method for Determining Pyrite, Bicarbonate, and Alkaline Earth Carbonates by Hammack in Proceedings of the Eighth Annual West Virginia Surface Mine Drainage Task Force Symposium, Apr. 7-8, 1987. Under those conditions any siderite (iron carbonate) present would have decomposed with the calcite (calcium carbonate) and dolomite (calcium magnesium carbonate). The work did not include siderite. However, the papers did show overlap between the decomposition temperatures of rhodochrosite and calcite.
The new thermal approach, described in this application, is being used to analyze some overburden samples that are of interest to Hammack at the DOE Federal Energy Technology Center.